Astronomers see the 1st stars dispel darkness 13 billion years ago at 'Cosmic Dawn'

Astronomers have used ground-based telescopes for the first time to peer back 13 billion years in time to observe the universe when the first stars first lifted the cosmic darkness.

This period, around 800 million years after the Big Bang, is known as "Cosmic Dawn," and it remains one of the most mysterious and important periods in the evolution of the universe.

This new glimpse of Cosmic Dawn comes courtesy of the Cosmology Large Angular Scale Surveyor (CLASS), an array of telescopes located high in the Atacama Desert region of Northern Chile. The primary mission of CLASS is to observe the Cosmic Microwave Background (CMB), a cosmic fossil left over from an event just after the Big Bang.

"People thought this couldn't be done from the ground. Astronomy is a technology-limited field, and microwave signals from the Cosmic Dawn are famously difficult to measure," team leader and Johns Hopkins professor of physics and astronomy, Tobias Marriage, said in a statement. "Ground-based observations face additional challenges compared to space. Overcoming those obstacles makes this measurement a significant achievement."

Let there be light ... Illuminating a cosmic fossil

Prior to around 380,000 years after the Big Bang, the infant universe would have seemed like a pretty dull place, visually at least. That is because during this period, light was unable to travel freely due to the fact that photons were endlessly scattered by free electrons.

This situation changed when the universe had expanded and cooled enough to allow electrons to bond with protons and create the first neutral atoms of hydrogen.

Suddenly, photons were free to travel unimpeded through the cosmos as the universe instantly went from transparent to opaque. This "first light" is seen today as the CMB.

When the first stars formed, their intense radiation ripped electrons from neutral hydrogen once again, an event called "reionization," turning the universe dark again during an epoch known as the "Cosmic Dark Ages."

The signal from Cosmic Dawn hunted by CLASS comes from the fingerprint of the universe's first stars within the CMB. This comes in the form of polarized microwave light around a million times fainter than standard cosmic microwaves.

As you may imagine, after travelling to us for 13 billion years and more, the light from Cosmic Dawn is extremely faint.

Trying to detect this polarized microwave light from Earth is extremely difficult because it is drowned out by natural events such as atmospheric changes and temperature fluctuations, as well as being obscured by human-made signals like radio waves, radar and satellite signals.

Thus, this cosmic radiation is usually only hunted from space by satellites like NASA's Wilkinson Microwave Anisotropy Probe (WMAP) and the European Space Agency's Planck space telescope. That was until CLASS.

CLASS Action

The team behind this new research compared data from CLASS with observations from Planck and WMAP. This allowed them to identify sources of interference and hone in on a signal from polarized microwave light in the CMB.

Polarization describes what happens when waves are oriented in the same direction. This can happen when light hits an object and scatters off it.

"When light hits the hood of your car and you see a glare, that's polarization. To see clearly, you can put on polarized glasses to take away glare," said team member Yunyang Li, who was a PhD student at Johns Hopkins and then a fellow at the University of Chicago while this research was being conducted. "Using the new common signal, we can determine how much of what we're seeing is cosmic glare from light bouncing off the hood of the Cosmic Dawn, so to speak."

What this team specifically aims to measure with CLASS is the probability of a photon from the CMB encountering an electron ripped free of neutral hydrogen by the universe's first stars and being scattered.

Doing this should help scientists better define signals from the CMB and the initial glow of the Big Bang, thus enabling them to paint a clear picture of the infant cosmos.

"Measuring this reionization signal more precisely is an important frontier of cosmic microwave background research," WMAP space mission team leader Charles Bennett said. "For us, the universe is like a physics lab. Better measurements of the universe help to refine our understanding of dark matter and neutrinos, abundant but elusive particles that fill the universe.

"By analyzing additional CLASS data going forward, we hope to reach the highest possible precision that's achievable."

This new research is built upon earlier work that saw CLASS map 75% of the night sky over Earth as it makes precise measurements of the polarization of the CMB.

"No other ground-based experiment can do what CLASS is doing," said Nigel Sharp, program director in the National Science Foundation (NSF) Division of Astronomical Sciences, supporter of CLASS since 2010. "The CLASS team has greatly improved measurement of the cosmic microwave polarization signal, and this impressive leap forward is a testament to the scientific value produced by NSF's long-term support."

The team's research was published on Wednesday (June 2) in The Astrophysical Journal.